Apparatus, systems, and methods for achieving lumbar facet fusion

ABSTRACT

Assemblies of one or more implant structures make possible the achievement of diverse interventions involving the fusion and/or stabilization of lumbar and sacral vertebra in a non-invasive manner, with minimal incision, and without the necessitating the removing the intervertebral disc. The representative lumbar spine interventions, which can be performed on adults or children, include, but are not limited to, lumbar interbody fusion; translaminar lumbar fusion; lumbar facet fusion; trans-iliac lumbar fusion; and the stabilization of a spondylolisthesis.

RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 11/136,141, filed May 24, 2005, which is acontinuation-in-part of U.S. patent application Ser. No. 10/914,629,filed Aug. 9, 2004 (now abandoned).

FIELD OF THE INVENTION

This application relates generally to the stabilization of the lumbarspine.

BACKGROUND OF THE INVENTION

The spine (see FIG. 1) is a complex interconnecting network of nerves,joints, muscles, tendons and ligaments, and all are capable of producingpain.

The spine is made up of small bones, called vertebrae. The vertebraeprotect and support the spinal cord. They also bear the majority of theweight put upon the spine.

Between each vertebra is a soft, gel-like “cushion,” called anintervertebral disc. These flat, round cushions act like shock absorbersby helping absorb pressure and keep the bones from rubbing against eachother. The intervertebral disc also binds adjacent vertebrae together.The intervertebral discs are a type of joint in the spine.Intervertebral disc joints can bend and rotate a bit but do not slide asdo most body joints.

Each vertebra has two other sets of joints, called facet joints (seeFIG. 2). The facet joints are located at the back of the spine(posterior). There is one facet joint on each lateral side (right andleft). One pair of facet joints faces upward (called the superiorarticular facet) and the other pair of facet joints faces downward(called the inferior articular facet). The inferior and superior facetjoints mate, allowing motion (articulation), and link vertebraetogether. Facet joints are positioned at each level to provide theneeded limits to motion, especially to rotation and to prevent forwardslipping (spondylolisthesis) of that vertebra over the one below.

In this way, the spine accommodates the rhythmic motions required byhumans to walk, run, swim, and perform other regular movements. Theintervetebral discs and facet joints stabilize the segments of the spinewhile preserving the flexibility needed to turn, look around, and getaround.

Degenerative changes in the spine can adversely affect the ability ofeach spinal segment to bear weight, accommodate movement, and providesupport. When one segment deteriorates to the point of instability, itcan lead to localized pain and difficulties. Segmental instabilityallows too much movement between two vertebrae. The excess movement ofthe vertebrae can cause pinching or irritation of nerve roots. It canalso cause too much pressure on the facet joints, leading toinflammation. It can cause muscle spasms as the paraspinal muscles tryto stop the spinal segment from moving too much. The instabilityeventually results in faster degeneration in this area of the spine.Degenerative changes in the spine can also lead to spondylolysis andspondylolisthesis. Spondylolisthesis is the term used to describe whenone vertebra slips forward on the one below it. This usually occursbecause there is a spondylolysis (defect) in the vertebra on top. Forexample, a fracture or a degenerative defect in the interarticular partsof lumbar vertebra L1 may cause a forward displacement of the lumbarvertebra L5 relative to the sacral vertebra S1 (called L5-S1pondylolisthesis). When a spondylolisthesis occurs, the facet joint canno longer hold the vertebra back. The intervertebral disc may slowlystretch under the increased stress and allow other upper vertebra toslide forward.

An untreated persistent, episodic, severely disabling back pain problemcan easily ruin the active life of a patient. In many instances, painmedication, splints, or other normally-indicated treatments can be usedto relieve intractable pain in a joint. However, in for severe andpersistent problems that cannot be managed by these treatment options,degenerative changes in the spine may require a bone fusion surgery tostop both the associated disc and facet joint problems.

A fusion is an operation where two bones, usually separated by a joint,are allowed to grow together into one bone. The medical term for thistype of fusion procedure is arthrodesis.

Lumbar fusion procedures have been used in the treatment of pain and theeffects of degenerative changes in the lower back. A lumbar fusion is afusion in the S1-L5-L4 region in the spine.

One conventional way of achieving a lumbar fusion is a procedure calledanterior lumbar interbody fusion (ALIF). In this procedure, the surgeonworks on the spine from the front (anterior) and removes a spinal discin the lower (lumbar) spine. The surgeon inserts a bone graft into thespace between the two vertebrae where the disc was removed (theinterbody space). The goal of the procedure is to stimulate thevertebrae to grow together into one solid bone (known as fusion). Fusioncreates a rigid and immovable column of bone in the problem section ofthe spine. This type of procedure is used to try and reduce back painand other symptoms.

Facet joint fixation procedures have also been used for the treatment ofpain and the effects of degenerative changes in the lower back. Theseprocedures take into account that the facet joint is the only truearticulation in the lumbosacral spine. In one conventional procedure forachieving facet joint fixation, the surgeon works on the spine from theback (posterior). The surgeon passes screws from the spinous processthrough the lamina and across the mid-point of one or more facet joints.

Conventional treatment of spondylolisthesis may include a laminectomy toprovide decompression and create more room for the exiting nerve roots.This can be combined with fusion using, e.g., an autologous fibulargraft, which may be performed either with or without fixation screws tohold the bone together. In some cases the vertebrae are moved back tothe normal position prior to performing the fusion, and in others thevertebrae are fused where they are after the slip, due to the increasedrisk of injury to the nerve with moving the vertebra back to the normalposition.

Currently, these procedures entail invasive open surgical techniques(anterior and/or posterior). Further, ALIF entails the surgical removalof the disc. Like all invasive open surgical procedures, such operationson the spine risk infections and require hospitalization. Invasive opensurgical techniques involving the spine continue to be a challenging anddifficult area.

SUMMARY OF THE INVENTION

The invention provides apparatus, systems, and methods for the fusionand/or stabilization of the lumbar spine. The apparatus, systems, andmethods include one or more elongated, stem-like implant structuressized and configured for the fusion or stabilization of adjacent bonestructures in the lumbar region of the spine, either across theintervertebral disc or across one or more facet joints. Each implantstructure includes a region formed along at least a portion of itslength to promote bony in-growth onto or into surface of the structureand/or bony growth entirely through all or a portion of the structure.The bony in-growth or through-growth region along the surface of theimplant structure accelerates bony in-growth or through-growth onto,into, or through the implant structure 20. The implant structuretherefore provides extra-articular/intra osseous fixation, when bonegrows in and around the bony in-growth or through-growth region. Bonyin-growth or through-growth onto, into, or through the implant structurehelps speed up the fusion and/or stabilization process of the adjacentbone regions fixated by the implant structure.

The assemblies of one or more implant structures make possible theachievement of diverse interventions involving the fusion and/orstabilization of lumbar and sacral vertebra in a non-invasive manner,with minimal incision, and without the necessitating the removing theintervertebral disc. The representative lumbar spine interventions,which can be performed on adults or children, include, but are notlimited to, lumbar interbody fusion; translaminar lumbar fusion; lumbarfacet fusion; trans-iliac lumbar fusion; and the stabilization of aspondylolisthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 are anatomic anterior and lateral views of a human spine.

FIG. 2 is an anatomic posterior perspective view of the lumbar region ofa human spine, showing lumbar vertebrae L2 to L5 and the sacralvertebrae.

FIG. 3 is an anatomic anterior perspective view of the lumbar region ofa human spine, showing lumbar vertebrae L2 to L5 and the sacralvertebrae.

FIG. 4 is a perspective view of a representative embodiment of anelongated, stem-like, cannulated implant structure well suited for thefusion or stabilization of adjacent bone structures in the lumbar regionof the spine, either across the intervertebral disc or across one ormore facet joints.

FIGS. 5 to 8 are perspective views of other representative embodimentsof implant structures well suited for the fusion or stabilization ofadjacent bone structures in the lumbar region of the spine, eitheracross the intervertebral disc or across one or more facet joints.

FIG. 9 is an anatomic anterior perspective view showing, in an explodedview prior to implantation, a representative configuration of anassembly of one or more implant structures as shown in FIG. 4, sized andconfigured to achieve anterior lumbar interbody fusion, in anon-invasive manner and without removal of the intervertebral disc.

FIG. 10 is an anatomic anterior perspective view showing the assemblyshown in FIG. 9 after implantation.

FIG. 11 is an anatomic right lateral perspective view showing theassembly shown in FIG. 9 after implantation.

FIG. 12 is an anatomic superior left lateral perspective view showingthe assembly shown in FIG. 9 after implantation.

FIGS. 13A to 13G are diagrammatic views showing, for purposes ofillustration, a representative lateral (or posterolateral) procedure forimplanting the assembly of implant structures shown in FIGS. 10 to 12.

FIG. 14 is an anatomic anterior perspective view showing, in an explodedview prior to implantation, assemblies comprising one or more implantstructures like that shown in FIG. 4 inserted from left and/or rightanterolateral regions of a given lumbar vertebra, in an angled paththrough the intervertebral disc and into an opposite anterolateralinterior region of the next inferior lumbar vertebra, FIG. 14 showing inparticular two implant structures entering on the right anterolateralside of L4, through the intervertebral disc and into the leftanterolateral region of L5, and one implant structure entering on theleft anterolateral side of L4, through the intervertebral disc and intothe right anterolateral region of L5, the left and right implantstructures crossing each other in transit through the intervertebraldisc.

FIG. 15 is an anatomic anterior perspective view showing, in an explodedview prior to implantation, assemblies comprising one or more implantstructures like that shown in FIG. 4 inserted from left and/or rightanterolateral regions of a given lumbar vertebra, in an angled paththrough the intervertebral disc and into an opposite anterolateralinterior region of the next inferior lumbar vertebra, FIG. 14 showing inparticular one implant structure entering on the right anterolateralside of L4, through the intervertebral disc and into the leftanterolateral region of L5, and one implant structure entering on theleft anterolateral side of L4, through the intervertebral disc and intothe right anterolateral region of L5, the left and right implantstructures crossing each other in transit through the intervertebraldisc.

FIG. 16 is an anatomic posterior perspective view, exploded prior toimplantation, of a representative configuration of an assembly of one ormore implant structures like that shown in FIG. 4, sized and configuredto achieve translaminar lumbar fusion in a non-invasive manner andwithout removal of the intervertebral disc.

FIG. 17 is an anatomic inferior transverse plane view showing theassembly shown in FIG. 16 after implantation.

FIG. 18 is an anatomic posterior perspective view, exploded prior toimplantation, of a representative configuration of an assembly of one ormore implant structures like that shown in FIG. 4, sized and configuredto achieve lumbar facet fusion, in a non-invasive manner and withoutremoval of the intervertebral disc.

FIG. 19 is an anatomic inferior transverse plane view showing theassembly shown in FIG. 18 after implantation.

FIG. 20 is an anatomic lateral view showing the assembly shown in FIG.18 after implantation.

FIG. 21A is an anatomic anterior perspective view showing, in anexploded view prior to implantation, a representative configuration ofan assembly of one or more implant structures like that shown in FIG. 4,sized and configured to achieve fusion between lumbar vertebra L5 andsacral vertebra S1, in a non-invasive manner and without removal of theintervertebral disc, using an anterior approach.

FIG. 21B is an anatomic anterior perspective view showing the assemblyshown in FIG. 21A after implantation.

FIG. 22A is an anatomic posterior view showing, in an exploded viewprior to implantation, another representative configuration of anassembly of one or more implant structures 20 sized and configured toachieve fusion between lumbar vertebra L5 and sacral vertebra S1, in anon-invasive manner and without removal of the intervertebral disc,using a postero-lateral approach entering from the posterior iliac spineof the ilium, angling through the SI-Joint, and terminating in thelumbar vertebra L5.

FIG. 22B is an anatomic posterior view showing the assembly shown inFIG. 22A after implantation.

FIG. 22C is an anatomic superior view showing the assembly shown in FIG.22B.

FIG. 23 is an anatomic laterial view showing a spondylolisthesis at theL5/S1 articulation, in which the lumbar vertebra L5 is displaced forward(anterior) of the sacral vertebra S1.

FIG. 24A is an anatomic anterior perspective view showing, in anexploded view prior to implantation, a representative configuration ofan assembly of one or more implant structures like that shown in FIG. 4,sized and configured to stabilize a spondylolisthesis at the L5/S1articulation.

FIG. 24B is an anatomic anterior perspective view showing the assemblyshown in FIG. 24A after implantation.

FIG. 24C is an anatomic lateral view showing the assembly shown in FIG.24B.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Although the disclosure hereof is detailed and exact to enable thoseskilled in the art to practice the invention, the physical embodimentsherein disclosed merely exemplify the invention that may be embodied inother specific structure. While the preferred embodiment has beendescribed, the details may be changed without departing from theinvention, which is defined by the claims.

I. The Implant Structure

FIG. 4 shows a representative embodiment of an elongated, stem-like,cannulated implant structure 20. As will be described in greater detaillater, the implant structure 20 is sized and configured for the fixationof bones which are to be fused (arthrodesed) (i.e. fixation of two ormore individual bones that are adjacent and/or jointed) and/or thestabilization of adjacent bone structures. In particular, and as will bedemonstrated, the implant structure is well suited for the fusion orstabilization of adjacent bone structures in the lumbar region of thespine, either across the intervertebral disc or across one or more facetjoints.

The implant structure 20 can be formed—e.g., by machining, molding, orextrusion—from a durable material usable in the prosthetic arts that isnot subject to significant bio-absorption or resorption by surroundingbone or tissue over time. The implant structure 20, is intended toremain in place for a time sufficient to stabilize a bone fracture orfusion site. Such materials include, but are not limited to, titanium,titanium alloys, tantalum, tivanium (aluminum, vanadium, and titanium),chrome cobalt, surgical steel, or any other total joint replacementmetal and/or ceramic, sintered glass, artificial bone, any uncementedmetal or ceramic surface, or a combination thereof.

Alternatively, the implant structure 20 may be formed from a suitabledurable biologic material or a combination of metal and biologicmaterial, such as a biocompatible bone-filling material. The implantstructure 20 may be molded from a flowable biologic material, e.g.,acrylic bone cement, that is cured, e.g., by UV light, to a non-flowableor solid material.

The implant structure 20 is sized according to the local anatomy. Themorphology of the local structures can be generally understood bymedical professionals using textbooks of human skeletal anatomy alongwith their knowledge of the site and its disease or injury. Thephysician is also able to ascertain the dimensions of the implantstructure 20 based upon prior analysis of the morphology of the targetedbone region using, for example, plain film x-ray, fluoroscopic x-ray, orMRI or CT scanning.

As FIGS. 5 to 8 show, the implant structure 20 can take various shapesand have various cross-sectional geometries. The implant structure 20can have, e.g., a generally curvilinear (i.e., round or oval)cross-section——as FIG. 5 shows for purposes of illustration—or agenerally rectilinear cross section (i.e., square or rectangular orhexagon or H-shaped or triangular—as FIG. 6 shows for purposes ofillustration—or combinations thereof. In FIG. 4, the implant structure20 is shown to be triangular in cross section, which effectively resistsrotation and micromotion once implanted.

As FIGS. 7 and 8 show, the implant structure 20, whether curvilinear(FIG. 7) or rectilinear (FIG. 8) can include a tapered region 34 atleast along a portion of its axial length, meaning that the width ordiameter of the implant structure 20 incrementally increases along itsaxial length. Desirably, the tapered region 34 corresponds with, in use,the proximal region of the implant structure 20 (i.e., the last part ofthe implant structure 20 to enter bone). The amount of the incrementalincrease in width or diameter can vary. As an example, for an implantstructure 20 having a normal diameter of 7 mm, the magnitude of theincremental increase at its maximum can range between about 0.25 mm to1.25 mm. The tapered region 34 enhances the creation and maintenance ofcompression between bone segments or regions.

As FIG. 4 shows, the implant structure 20 includes a region 24 formedalong at least a portion of its length to promote bony in-growth onto orinto surface of the structure and/or bony growth entirely through all ora portion of the structure. The bony in-growth or through-growth region24 along the surface of the implant structure 20 accelerates bonyin-growth or through-growth onto, into, or through the implant structure20. Bony in-growth or through-growth onto, into, or through the implantstructure 20 helps speed up the fusion process of the adjacent boneregions fixated by the implant structure 20.

The bony in-growth or through-growth region 24 desirably extends alongthe entire outer surface of the implant structure 20, as shown in FIGS.4 to 8. The bony in-growth region 24 or through-growth can comprise,e.g., through holes, and/or various surface patterns, and/or varioussurface textures, and/or pores, or combinations thereof. Theconfiguration of the bony in-growth or through-growth region 24 can, ofcourse, vary. By way of examples, the bony in-growth or through-growthregion 24 can comprise an open mesh configuration; or beadedconfiguration; or a trabecular configuration; or include holes orfenestrations. Any configuration conducive to bony in-growth and/or bonythrough-growth will suffice.

The bony in-growth or through-growth region 24 can be coated or wrappedor surfaced treated to provide the bony in-growth or through-growthregion, or it can be formed from a material that itself inherentlypossesses a structure conducive to bony in-growth or through-growth,such as a porous mesh, hydroxyapetite, or other porous surface. The bonyin-growth or through-growth region can includes holes that allow bone togrow throughout the region.

In a preferred embodiment, the bony in-growth region or through-growthregion 24 comprises a porous plasma spray coating on the implantstructure 20. This creates a biomechanically rigorous fixation/fusionsystem, designed to support reliable fixation/fusion and acute weightbearing capacity.

The bony in-growth or through-growth region 24 may further be coveredwith various other coatings such as antimicrobial, antithrombotic, andosteoinductive agents, or a combination thereof. The entire implantstructure 20 may be impregnated with such agents, if desired.

The implant structure includes an interior bore that accommodates itsplacement in a non-invasive manner by sliding over a guide pin, as willbe described in greater detail later.

As before stated, the implant structure 20 is well suited for the fusionand/or stabilization of adjacent bone structures in the lumbar region ofthe spine. Representative examples of the placement of the implantstructure 20 in the lumbar region of the spine will now be described.

A. Use of the Implant Structures to Achieve Anterior Lumbar InterbodyFusion

FIG. 9 shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to achieve anterior lumbar interbodyfusion, in a non-invasive manner and without removal of theintervertebral disc. FIGS. 10 to 12 show the assembly afterimplantation, respectively, in an anterior view, a right lateral view,and a superior left lateral perspective view.

In the representative embodiment illustrated in FIGS. 10 to 12, theassembly comprises three implant structures 20. It should beappreciated, however, that a given assembly can include a greater orlesser number of implant structures 20.

In the representative embodiment shown in FIGS. 10 to 12, the threeimplant structures 20 are spaced in an adjacent lateral array. Theimplant structures 20 extend from an anterolateral region of a selectedvertebral body (i.e., a lateral region anterior to a transverseprocess), across the intervertebral disc into an opposite anterolateralregion of an adjacent caudal (inferior) vertebra. As shown in FIGS. 10to 12, the array of implant structures 20 extends in an angled path(e.g., about 20° to about 40° off horizontal) through the cranial(superior) lumbar vertebral body (shown as L4) in an inferior direction,through the adjoining intervertebral disc, and terminates in the nextadjacent caudal (inferior) lumbar vertebral body (shown as L5).

More particularly, in the representative embodiment shown in FIGS. 9 to12, the implant structures 20 enter the right anterolateral region ofvertebra L4 and terminate within the left anterolateral interior ofvertebra L5, spanning the intervertebral disc between L4 and L5.

Alternatively, or in combination, an array of implant structures 20 canlikewise extend between L5 and S1 in the same trans-disc formation.

The implant structures 20 are sized according to the local anatomy. Theimplant structures 20 can be sized differently, e.g., 3 mm, 4 mm, 6 mm,etc.), to accommodate anterolateral variations in the anatomy. Theimplant structures 20 can be sized for implantation in adults orchildren.

The intimate contact created between the bony in-growth orthrough-growth region 24 along the surface of the implant structure 20accelerates bony in-growth or through-growth onto, into, or through theimplant structure 20, to accelerate trans-disc fusion between theselumbar vertebrae.

FIGS. 13A to 13G diagrammatically show, for purposes of illustration, arepresentative lateral (or posterolateral) procedure for implanting theassembly of implant structures 20 shown in FIGS. 10 to 12.

The physician identifies the vertebrae of the lumbar spine region thatare to be fused using, e.g., the Faber Test, or CT-guided injection, orX-ray/MRI of the lumbar spine. Aided by lateral and anterior-posterior(A-P) c-arms, and with the patient lying in a prone position (on theirstomach), the physician makes a 3 mm incision laterally orposterolaterally from the side (see FIG. 13A). Aided by conventionalvisualization techniques, e.g., using X-ray image intensifiers such as aC-arms or fluoroscopes to produce a live image feed which is displayedon a TV screen, a guide pin 38 is introduced by conventional means intoL4 (see FIG. 13B) for the first, most anterolateral implant structure(closest to the right transverse process of L4), in the desired angledinferiorly-directed path through the intervertebral disc and into theinterior left anterolateral region of vertebra L5.

When the guide pin 38 is placed in the desired orientation, thephysician desirable slides a soft tissue protector over the guide pin 38before proceeding further. To simplify the illustration, the soft tissueprotector is not shown in the drawings.

Through the soft tissue protector, a cannulated drill bit 40 is nextpassed over the guide pin 38 (see FIG. 13C). The cannulated drill bit 40forms a pilot insertion path or bore 42 along the first angled pathdefined by the guide pin 38. A single drill bit or multiple drill bits40 can be employed to drill through bone fragments or bone surfaces tocreate a pilot bore 42 of the desired size and configuration.

When the pilot bore 42 is completed, the cannulated drill bit 40 iswithdrawn over the guide pin 38.

Through the soft tissue protector, a broach 44 having the externalgeometry and dimensions matching the external geometry and dimensions ofthe implant structure 20 (which, in the illustrated embodiment, istriangular) (see FIG. 13D) is tapped through the soft tissue protectorover the guide pin 38 and into the pilot bore 42. The shaped broach 44cuts along the edges of the pilot bore 42 to form the desired profile(which, in the illustrated embodiment, is triangular) to accommodate theimplant structure 20.

The broach 44 is withdrawn (see FIG. 13E), and the first, mostanterolateral implant structure 20 is passed over the guide pin 38through the soft tissue protector into the broached bore 48. The guidepin 38 and soft tissue protector are withdrawn from the first implantstructure 20.

The physician repeats the above-described procedure sequentially for thenext anterolateral implant structures 20: for each implant structure,inserting the guide pin 38, forming the pilot bore, forming the broachedbore, inserting the respective implant structure, withdrawing the guidepin, and then repeating the procedure for the next implant structure,and so on until all implant structures 20 are placed (as FIGS. 13F and13G indicate). The incision site(s) are closed.

In summary, the method for implanting the assembly of the implantstructures 20 comprises (i) identifying the bone structures to be fusedand/or stabilized; (ii) opening an incision; (iii) using a guide pin toestablished a desired implantation path through bone for the implantstructure 20; (iv) guided by the guide pin, increasing the cross sectionof the path; (v) guided by the guide pin, shaping the cross section ofthe path to correspond with the cross section of the implant structure20; (vi) inserting the implant structure 20 through the path over theguide pin; (vii) withdrawing the guide pin; (viii) repeating, asnecessary, the procedure sequentially for the next implant structure(s)until all implant structures 20 contemplated are implanted; and (ix)closing the incision.

As FIGS. 14 and 15 show, assemblies comprising one or more implantstructures 20 can be inserted from left and/or right anterolateralregions of a given lumbar vertebra, in an angled path through theintervertebral disc and into an opposite anterolateral interior regionof the next inferior lumbar vertebra.

For purposes of illustration, FIG. 14 shows two implant structures 20entering on the right anterolateral side of L4, through theintervertebral disc and into the left anterolateral region of L5, andone implant structure 20 entering on the left anterolateral side of L4,through the intervertebral disc and into the right anterolateral regionof L5. In this arrangement, the left and right implant structures 20cross each other in transit through the intervertebral disc.

As another illustration of a representative embodiment, FIG. 15 showsone implant structure 20 entering on the right anterolateral side of L4,through the intervertebral disc and into the left anterolateral regionof L5, and one implant structure 20 entering on the left anterolateralside of L4, through the intervertebral disc and into the rightanterolateral region of L5. In this arrangement as well, the left andright implant structures 20 cross each other in transit through theintervertebral disc.

B. Use of Implant Structures to Achieve Translaminal Lumbar Fusion(Posterior Approach)

FIG. 16 shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to achieve translaminar lumbar fusionin a non-invasive manner and without removal of the intervertebral disc.FIG. 17 shows the assembly after implantation, respectively, in aninferior transverse plane view.

As can be seen in the representative embodiment illustrated in FIGS. 16and 17, the assembly comprises two implant structures 20. The firstimplant structure 20 extends from the left superior articular process ofvertebra L5, through the adjoining facet capsule into the left inferiorarticular process of vertebra L4, and, from there, further through thelamina of vertebra L4 into an interior right posterolateral region ofvertebra L4 adjacent the spinous process. The second implant structure20 extends from the right superior articular process of vertebra L5,through the adjoining facet capsule into the right inferior articularprocess of vertebra L4, and, from there, further through the lamina ofvertebra L4 into an interior left posterolateral region of vertebra L4adjacent the spinous process. The first and second implant structures 20cross each other within the medial lamina of vertebra L4.

The first and second implant structures 20 are sized and configuredaccording to the local anatomy. The selection of a translaminar lumbarfusion (posterior approach) is indicated when the facet joints arealigned with the sagittal plane. Removal of the intervertebral disc isnot required, unless the condition of the disc warrants its removal.

A procedure incorporating the technical features of the procedure shownin FIGS. 13A to 13G can be tailored to a posterior procedure forimplanting the assembly of implant structures 20 shown in FIGS. 16 and17. The method comprises (i) identifying the vertebrae of the lumbarspine region that are to be fused; (ii) opening an incision, whichcomprises, e.g., with the patient lying in a prone position (on theirstomach), making a 3 mm posterior incision; and (iii) using a guide pinto established a desired implantation path through bone for the first(e.g., left side) implant structure 20, which, in FIGS. 16 and 17,traverses through the left superior articular process of vertebra L5,through the adjoining facet capsule into the left inferior articularprocess of vertebra L4, and then through the lamina of vertebra L4 intoan interior right posterolateral region of vertebra L4 adjacent thespinous process. The method further includes (iv) guided by the guidepin, increasing the cross section of the path; (v) guided by the guidepin, shaping the cross section of the path to correspond with the crosssection of the implant structure; (vi) inserting the implant structure20 through the path over the guide pin; (vii) withdrawing the guide pin;and (viii) using a guide pin to established a desired implantation paththrough bone for the second (e.g., right side) implant structure 20,which, in FIGS. 16 and 17, traverses through the right superiorarticular process of vertebra L5, through the adjoining facet capsuleinto the right inferior articular process of vertebra L4, and throughthe lamina of vertebra L4 into an interior left posterolateral region ofvertebra L4 adjacent the spinous process. The physician repeats theremainder of the above-described procedure sequentially for the rightimplant structure 20 as for the left, and, after withdrawing the guidepin, closes the incision.

The intimate contact created between the bony in-growth orthrough-growth region 24 along the surface of the implant structure 20across the facet joint accelerates bony in-growth or through-growthonto, into, or through the implant structure 20, to accelerate fusion ofthe facets joints between L4 and L5. Of course, translaminar lumbarfusion between L5 and S1 can be achieved using first and second implantstructures in the same manner.

C. Use of Implant Structures to Achieve Lumbar Facet Fusion (PosteriorApproach)

FIG. 18 shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to lumbar facet fusion, in anon-invasive manner and without removal of the intervertebral disc.FIGS. 19 and 20 show the assembly after implantation, respectively, inan inferior transverse plane view and a lateral view.

As can be seen in the representative embodiment illustrated in FIGS. 18and 20, the assembly comprises two implant structures 20. The firstimplant structure 20 extends from the left inferior articular process ofvertebra L4, through the adjoining facet capsule into the left superiorarticular process of vertebra L5 and into the pedicle of vertebra L5.The second implant structure 20 extends from the right inferiorarticular process of vertebra L5, through the adjoining facet capsuleinto the right superior articular process of vertebra L5 and into thepedicle of vertebra L5. In this arrangement, the first and secondimplant structures 20 extend in parallel directions on the left andright pedicles of vertebra L5. The first and second implant structures20 are sized and configured according to the local anatomy. Theselection of lumbar facet fusion (posterior approach) is indicated whenthe facet joints are coronally angled. Removal of the intervertebraldisc is not necessary, unless the condition of the disc warrants itsremoval.

A procedure incorporating the technical features of the procedure shownin FIGS. 13A to 13G can be tailored to a posterior procedure forimplanting the assembly of implant structures 20 shown in FIGS. 18 to20. The method comprises (i) identifying the vertebrae of the lumbarspine region that are to be fused; (ii) opening an incision, whichcomprises, e.g., with the patient lying in a prone position (on theirstomach), making a 3 mm posterior incision; and (iii) using a guide pinto established a desired implantation path through bone for the first(e.g., left side) implant structure 20, which, in FIGS. 18 to 20,traverses through the left inferior articular process of vertebra L4,through the adjoining facet capsule into the left superior articularprocess of vertebra L5 and into the pedicle of vertebra L5. The methodfurther includes (iv) guided by the guide pin, increasing the crosssection of the path; (v) guided by the guide pin, shaping the crosssection of the path to correspond with the cross section of the implantstructure 20; (vi) inserting the implant structure 20 through the pathover the guide pin; (vii) withdrawing the guide pin; and (viii) using aguide pin to established a desired implantation path through bone forthe second (e.g., right side) implant structure 20, which, in FIGS. 18to 20, traverses through the right inferior articular process ofvertebra L5, through the adjoining facet capsule into the right superiorarticular process of vertebra L5 and into the pedicle of vertebra L5.The physician repeats the remainder of the above-described proceduresequentially for the right implant structure 20 as for the left and,withdrawing the guide pin, closes the incision.

The intimate contact created between the bony in-growth orthrough-growth region 24 along the surface of the implant structure 20across the facet joint accelerates bony in-growth or through-growthonto, into, or through the implant structure 20, to accelerate fusion ofthe facets joints between L4 and L5.

Of course, translaminar lumbar fusion between L5 and S1 can be achievedusing first and second implant structures in the same manner.

D. Use of Implant Structures to Achieve Trans-Iliac Lumbar Fusion(Anterior Approach)

FIG. 21A shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to achieve fusion between lumbarvertebra L5 and sacral vertebra S1, in a non-invasive manner and withoutremoval of the intervertebral disc. FIG. 21B shows the assembly afterimplantation.

In the representative embodiment illustrated in FIGS. 21A and 21B, theassembly comprises two implant structures 20. It should be appreciated,however, that a given assembly can include a greater or lesser number ofimplant structures 20.

As FIGS. 21A and 21B show, the assembly comprises two implant structures20 inserted from left and right anterolateral regions of lumbar vertebraL5, in an angled path (e.g., about 20° to about 40° off horizontal)through the intervertebral disc in an inferior direction, into andthrough opposite anterolateral interior regions of sacral vertebra S1,through the sacro-iliac joint, and terminating in the ilium. In thisarrangement, the left and right implant structures 20 cross each otherin transit through the intervertebral disc. As before described, theimplant structures 20 are sized according to the local anatomy.

The intimate contact created between the bony in-growth orthrough-growth region 24 along the surface of the implant structure 20accelerates bony in-growth or through-growth onto, into, or through theimplant structure 20, to accelerate lumbar trans-iliac fusion betweenvertebra L5 and S1.

A physician can employ the lateral (or posterolateral) procedure asgenerally shown in FIGS. 13A to 13G for implanting the assembly ofimplant structures 20 shown in FIGS. 21A and 21B, including forming apilot bore over a guide pin inserted in the angled path, forming abroached bore, inserting the right implant 20 structure, withdrawing theguide pin, and repeating for the left implant structure 20, or viceversa. The incision site(s) are closed.

The assembly as described makes possible the achievement of trans-iliaclumbar fusion using an anterior in a non-invasive manner, with minimalincision, and without necessarily removing the intervertebral discbetween L5 and S1.

E. Use of Implant Structures to Achieve Trans-Iliac Lumbar Fusion(Postero-Lateral Approach From Posterior Iliac Spine)

FIG. 22A shows, in an exploded view prior to implantation, anotherrepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to achieve fusion between lumbarvertebra L5 and sacral vertebra S1, in a non-invasive manner and withoutremoval of the intervertebral disc. FIGS. 22B and 22C show the assemblyafter implantation.

As FIGS. 22A and 22B show, the one or more implant structures areintroduced in a postero-lateral approach entering from the posterioriliac spine of the ilium, angling through the SI-Joint into and throughthe sacral vertebra S1, and terminating in the lumbar vertebra L5. Thispath and resulting placement of the implant structures 20 are also shownin FIG. 22C. In the illustrated embodiment, two implant structures 20are placed in this manner, but there can be more or fewer implantstructures 20. Also in the illustrated embodiment, the implantstructures 20 are triangular in cross section, but it should beappreciated that implant structures 20 of other cross sections aspreviously described can be used.

The postero-lateral approach involves less soft tissue disruption thatthe lateral approach, because there is less soft tissue overlying theentry point of the posterior iliac spine of the ilium. Introduction ofthe implant structure 20 from this region therefore makes possible asmaller, more mobile incision.

The set-up for a postero-lateral approach is generally the same as for alateral approach. It desirably involves the identification of the lumbarregion that is to be fixated or fused (arthrodesed) using, e.g., theFaber Test, or CT-guided injection, or X-ray/MRI of SI Joint. It isdesirable performed with the patient lying in a prone position (on theirstomach) and is aided by lateral and anterior-posterior (A-P) c-arms.The same surgical tools are used to form the pilot bore over a guide pin(e.g., on the right side), except the path of the pilot bore now startsfrom the posterior iliac spine of the ilium, angles through theSI-Joint, and terminates in the lumbar vertebra L5. The broached bore isformed, and the right implant 20 structure is inserted. The guide pin iswithdrawn, and the procedure is repeated for the left implant structure20, or vice versa. The incision site(s) are closed.

The assembly as described makes possible the achievement of trans-iliaclumbar fusion using a postero-lateral approach in a non-invasive manner,with minimal incision, and without necessarily removing theintervertebral disc between L5 and S1.

F. Use of Implant Structures to Stabilize a Spondylolisthesis

FIG. 23 shows a spondylolisthesis at the L5/S1 articulation, in whichthe lumbar vertebra L5 is displaced forward (anterior) of the sacralvertebra S1. As FIG. 23 shows, the posterior fragment of L5 remains innormal relation to the sacrum, but the anterior fragment and the L5vertebral body has moved anteriorly. Spondylolisthesis at the L5/S1articulation can result in pressure in the spinal nerves of the caudaequine as they pass into the superior part of the sacrum, causing backand lower limb pain.

FIG. 24A shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to stabilize the spondylolisthesis atthe L5/S1 articulation. FIGS. 24B and 24C show the assembly afterimplantation.

As shown, the implant structure 20 extends from a posterolateral regionof the sacral vertebra S1, across the intervertebral disc into anopposite anterolateral region of the lumbar vertebra L5. The implantstructure 20 extends in an angled path (e.g., about 20° to about 40° offhorizontal) through the sacral vertebra S1 in a superior direction,through the adjoining intervertebral disc, and terminates in the lumbarvertebra L5.

A physician can employ a posterior approach for implanting the implantstructure 20 shown in FIGS. 24A, 24B, and 24C, which includes forming apilot bore over a guide pin inserted in the angled path from theposterior of the sacral vertebra S1 through the intervertebral disc andinto an opposite anterolateral region of the lumbar vertebra L5, forminga broached bore, inserting the implant structure 20, and withdrawing theguide pin. The incision site is then closed. As previously described,more than one implant structure 20 can be placed in the same manner tostabilize a spondylolisthesis. Furthermore, a physician can fixate theimplant structure(s) 20 using the anterior trans-iliac lumbar path, asshown in FIG. 21A/B or 22A/B/C.

The physician can, if desired, combine stabilization of thespondylolisthesis, as shown in FIG. 24A/B/C, with a reduction,realigning L5 and S-1. The physician can also, if desired, combinestabilization of the spondylolisthesis, as shown in FIG. 24A/B/C (withor without reduction of the spondylolisthesis), with a lumbar facetfusion, as shown in FIGS. 18 to 20. The physician can also, if desired,combine stabilization of the spondylolisthesis, as shown in FIG.24A/B/C, with a decompression, e.g., by the posterior removal of thespinous process and laminae bilaterally.

II. Conclusion

The various representative embodiments of the assemblies of the implantstructures 20, as described, make possible the achievement of diverseinterventions involving the fusion and/or stabilization of lumbar andsacral vertebra in a non-invasive manner, with minimal incision, andwithout the necessitating the removing the intervertebral disc. Therepresentative lumbar spine interventions described can be performed onadults or children and include, but are not limited to, lumbar interbodyfusion; translaminar lumbar fusion; lumbar facet fusion; trans-iliaclumbar fusion; and the stabilization of a spondylolisthesis. It shouldbe appreciated that such interventions can be used in combination witheach other and in combination with conventional fusion/fixationtechniques to achieve the desired therapeutic objectives.

Significantly, the various assemblies of the implant structures 20 asdescribed make possible lumbar interbody fusion without the necessity ofremoving the intervertebral disc. For example, in conventional anteriorlumbar interbody fusion procedures, the removal of the intervertebraldisc is a prerequisite of the procedure. However, when using theassemblies as described to achieve anterior lumbar interbody fusion,whether or not the intervertebral disc is removed depends upon thecondition of the disc, and is not a prerequisite of the procedureitself. If the disc is healthy and has not appreciably degenerated, oneor more implant structures 20 can be individually inserted in aminimally invasive fashion, across the intervertebral disc in the lumbarspine area, leaving the disc intact.

In all the representative interventions described, the removal of adisc, or the scraping of a disc, is at the physician's discretion, basedupon the condition of the disc itself, and is not dictated by theprocedure. The bony in-growth or through-growth regions 24 of theimplant structures 20 described provide both extra-articular and intraosseous fixation, when bone grows in and around the bony in-growth orthrough-growth regions 24.

Conventional tissue access tools, obturators, cannulas, and/or drillscan be used during their implantation. No disc preparation, removal ofbone or cartilage, or scraping are required before and during formationof the insertion path or insertion of the implant structures 20, so aminimally invasive insertion path sized approximately at or about themaximum outer diameter of the implant structures 20 need be formed.Still, the implant structures 20, which include the elongated bonyin-growth or through-growth regions 24, significantly increase the sizeof the fusion area, from the relatively small surface area of a givenjoint between adjacent bones, to the surface area provided by anelongated bony in-growth or through-growth regions 24. The implantstructures 20 can thereby increase the surface area involved in thefusion and/or stabilization by 3-fold to 4-fold, depending upon thejoint involved.

The implant structures 20 can obviate the need for autologous grafts,bone graft material, additional pedicle screws and/or rods, hollowmodular anchorage screws, cannulated compression screws, cages, orfixation screws. Still, in the physician's discretion, bone graftmaterial and other fixation instrumentation can be used in combinationwith the implant structures 20.

The implant structures 20 make possible surgical techniques that areless invasive than traditional open surgery with no extensive softtissue stripping and no disc removal. The assemblies make possiblestraightforward surgical approaches that complement the minimallyinvasive surgical techniques. The profile and design of the implantstructures 20 minimize rotation and micro-motion. Rigid implantstructures 20 made from titanium provide immediate post-op fusionstability. A bony in-growth region 24 comprising a porous plasma spraycoating with irregular surface supports stable bone fixation/fusion. Theimplant structures 20 and surgical approaches make possible theplacement of larger fusion surface areas designed to maximizepost-surgical weight bearing capacity and provide a biomechanicallyrigorous implant designed specifically to stabilize the heavily loadedlumbar spine.

The foregoing is considered as illustrative only of the principles ofthe invention. Furthermore, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and operation shown anddescribed. While the preferred embodiment has been described, thedetails may be changed without departing from the invention, which isdefined by the claims.

I claim:
 1. A method for translaminal lumbar fusion comprising creatingan insertion path that extends from a superior articular process of aselected lumbar vertebra, cranially through the adjoining facet capsuleinto a corresponding inferior articular process of an adjacent lumbarvertebra, and, from there, further through the lamina of the adjacentvertebra into an interior opposite posterolateral region adjacent thespinous process of the adjacent vertebra, providing a bone fixationimplant comprises an elongated implant structure having a rectilinearcross section including an exterior surface region treated to providebony in-growth or through-growth along the implant structure, andinserting the bone fixation implant through the insertion path from thesuperior articular process of the selected lumbar vertebra, craniallythrough the adjoining facet capsule into the inferior articular processof the adjacent lumbar vertebra, and, from there, further through thelamina of the adjacent vertebra into an interior opposite posterolateralregion adjacent the spinous process of the adjacent vertebra.
 2. Amethod according to claim 1 wherein the elongated implant structure hasa triangular cross section.
 3. A method according to claim 1 wherein thebony in-growth region or through-growth region comprises a porous plasmaspray coating on the implant structure.
 4. A method according to claim 1wherein the insertion path is created in a non-invasive manner.
 5. Amethod according to claim 1 wherein the insertion path comprises a boresized approximately at or approximately about an outer maximum dimensionof the bone fixation implant.
 6. A method according to claim 1 whereinat least a first implant structure extends from a superior articularprocess of the selected lumbar vertebra, in a cranial direction throughthe adjoining facet capsule into the corresponding inferior articularprocess of the adjacent lumbar vertebra, and, from there, furtherthrough the lamina of the adjacent lumbar vertebra into an interioropposite posterolateral region adjacent the spinous process of theadjacent lumbar vertebra, and wherein at least a second implantstructure extends from an opposite superior articular process of theselected lumbar vertebra, through the adjoining facet capsule into thecorresponding inferior articular process of the adjacent vertebra, and,from there, further through the lamina of the adjacent lumbar vertebrainto an interior opposite posterolateral region adjacent the spinousprocess of the adjacent lumbar vertebra, the first and second implantstructures crossing each other within the medial lamina of adjacentlumbar vertebra.
 7. A method according to claim 1 wherein the insertionpath extends from the superior articular process of lumbar vertebra L5,cranially through the adjoining facet capsule into an inferior articularprocess of the lumbar vertebra L4, and, from there, further through thelamina of the lumbar vertebra L4 into an interior oppositeposterolateral region adjacent the spinous process of the lumbarvertebra L4.
 8. A method according to claim 1 wherein at least a firstimplant structure extends from a left superior articular process of thelumbar vertebra L5, in a cranial direction through the adjoining facetcapsule into the corresponding left inferior articular process of theadjacent lumbar vertebra L4, and, from there, further through the laminaof the adjacent lumbar vertebra L4 into an interior right posterolateralregion adjacent the spinous process of the adjacent lumbar vertebra L4,and wherein at least a second implant structure extends from a rightsuperior articular process of the selected lumbar vertebra L5, throughthe adjoining facet capsule into the corresponding right inferiorarticular process of the adjacent lumbar vertebra L4, and, from there,further through the lamina of the adjacent lumbar vertebra L4 into aninterior left posterolateral region adjacent the spinous process of theadjacent lumbar vertebra L4, the first and second implant structurescrossing each other within the medial lamina of adjacent lumbar vertebraL4.
 9. A system comprising at least one bone fixation implant comprisingan elongated implant structure having a rectilinear cross sectionincluding an exterior surface region treated to provide bony in-growthor through-growth along the implant structure inserted through aninsertion path from a superior articular process of a selected lumbarvertebra, cranially through the adjoining facet capsule into thecorresponding inferior articular process of the adjacent lumbarvertebra, and, from there, further through the lamina of the adjacentvertebra into an interior opposite posterolateral region adjacent thespinous process of the adjacent vertebra to affect translaminal lumbarfusion.
 10. A system according to claim 9 wherein the elongated implantstructure has a triangular cross section.
 11. A system according toclaim 10 wherein the bony in-growth region or through-growth regioncomprises a porous plasma spray coating on the implant structure.
 12. Asystem comprising at least a first implant structure comprising anelongated implant structure having a rectilinear cross section includingan exterior surface region treated to provide bony in-growth orthrough-growth along the implant structure inserted through an insertionpath from a superior articular process of the selected lumbar vertebra,in a cranial direction through the adjoining facet capsule into thecorresponding inferior articular process of the adjacent lumbarvertebra, and, from there, further through the lamina of the adjacentlumbar vertebra into an interior opposite posterolateral region adjacentthe spinous process of the adjacent lumbar vertebra, and at least asecond implant structure comprising an elongated implant structurehaving a rectilinear cross section including an exterior surface regiontreated to provide bony in-growth or through-growth along the implantstructure inserted through an insertion path from an opposite superiorarticular process of the selected lumbar vertebra, through the adjoiningfacet capsule into the corresponding inferior articular process of theadjacent vertebra, and, from there, further through the lamina of theadjacent lumbar vertebra into an interior opposite posterolateral regionadjacent the spinous process of the adjacent lumbar vertebra, the firstand second implant structures crossing each other within the mediallamina of adjacent lumbar vertebra to affect translaminal lumbar fusion.13. A system according to claim 12 wherein the elongated implantstructure has a triangular cross section.
 14. A system according toclaim 12 wherein the bony in-growth region or through-growth regioncomprises a porous plasma spray coating on the implant structure.
 15. Amethod for lumbar facet fusion comprising creating an insertion paththat extends from an inferior articular process of a selected lumbarvertebra, in a caudal direction through the adjoining facet capsule intoa corresponding superior articular process of an adjacent lumbarvertebra and into a pedicle of the adjacent lumbar vertebra, providing abone fixation implant comprising an elongated implant structure having alongitudinal axis and a rectilinear cross section transverse to thelongitudinal axis and including an exterior surface region treated toprovide bony in-growth or through-growth along the implant structure,and inserting the bone fixation implant through the insertion path fromthe inferior articular process of the selected lumbar vertebra, in acaudal direction through the adjoining facet capsule into thecorresponding superior articular process of the adjacent lumbar vertebraand into a pedicle of the adjacent lumbar vertebra.
 16. A methodaccording to claim 15 wherein the elongated implant structure has atriangular cross section.
 17. A method according to claim 15 wherein thebony in-growth region or through-growth region comprises a porous plasmaspray coating on the implant structure.
 18. A method according to claim15 wherein the insertion path is created in a non-invasive manner.
 19. Amethod according to claim 15 wherein the insertion path comprises a boresized approximately at or approximately about an outer maximum dimensionof the bone fixation implant.
 20. A method according to claim 15 whereinat least a first implant structure extends from a left inferiorarticular process of the selected lumbar vertebra, in a caudal directionthrough the adjoining facet capsule into the corresponding superiorarticular process of the adjacent lumbar vertebra and into a pedicle ofthe adjacent lumbar vertebra, and wherein at least a second implantstructure extends from a right inferior articular process of theselected lumbar vertebra, in a caudal direction through the adjoiningfacet capsule into the corresponding superior articular process of theadjacent lumbar vertebra and into a pedicle of the adjacent lumbarvertebra.
 21. A method according to claim 15 wherein at least a firstimplant structure extends from an inferior articular process of thelumbar vertebra L4, in a caudal direction through the adjoining facetcapsule into the corresponding superior articular process of theadjacent lumbar vertebra L5 and into a pedicle of the adjacent lumbarvertebra L5.
 22. A method according to claim 15 wherein at least a firstimplant structure extends from a left inferior articular process of thelumbar vertebra L4, in a caudal direction through the adjoining facetcapsule into the corresponding superior articular process of theadjacent lumbar vertebra L5 and into a pedicle of the adjacent lumbarvertebra L5, and wherein at least a second implant structure extendsfrom a right inferior articular process of the selected lumbar vertebraL4, in a caudal direction through the adjoining facet capsule into thecorresponding superior articular process of the adjacent lumbar vertebraL5 and into a pedicle of the adjacent lumbar vertebra L5.
 23. A systemcomprising a bone fixation implant comprising an elongated implantstructure having a longitudinal axis and a rectilinear cross sectiontransverse to the longitudinal axis and including an exterior surfaceregion treated to provide bony in-growth or through-growth along theimplant structure inserted through an insertion path from an inferiorarticular process of a selected lumbar vertebra, in a caudal directionthrough the adjoining facet capsule into a corresponding superiorarticular process of an adjacent lumbar vertebra and into a pedicle ofthe adjacent lumbar vertebra to affect lumbar facet fusion.
 24. A systemaccording to claim 23 wherein the elongated implant structure has atriangular cross section.
 25. A system according to claim 23 wherein thebony in-growth region or through-growth region comprises a porous plasmaspray coating on the implant structure.
 26. A system comprising at leasta first implant structure comprising an elongated implant structurehaving a longitudinal axis and a rectilinear cross section transverse tothe longitudinal axis and including an exterior surface region treatedto provide bony in-growth or through-growth along the implant structurethat extends from a left inferior articular process of a selected lumbarvertebra, in a caudal direction through the adjoining facet capsule intoa corresponding superior articular process of an adjacent lumbarvertebra and into a pedicle of the adjacent lumbar vertebra, and atleast a second implant structure comprising an elongated implantstructure having a rectilinear cross section including an exteriorsurface region treated to provide bony in-growth or through-growth alongthe implant structure that extends from a right inferior articularprocess of the selected lumbar vertebra, in a caudal direction throughthe adjoining facet capsule into the corresponding superior articularprocess of the adjacent lumbar vertebra and into a pedicle of theadjacent lumbar vertebra, the first and second implant structuresaffecting lumbar facet fusion.
 27. A system according to claim 26wherein the elongated implant structure has a triangular cross section.28. A system according to claim 26 wherein the bony in-growth region orthrough-growth region comprises a porous plasma spray coating on theimplant structure.